Pandemics of influenza occur at ten to fifteen year intervals causing an enormous toll of human life and economic loss on a world-wide scale. These pandemics result because the major surface antigen of influenza virus, the hemagglutinin, undergoes complete alteration of its antigenicity, permitting the virus to evade host immunity. Peptide maps of tryptic hydrolysates of the hemagglutinin isolated from different antigenic subtypes show dramatic differences among them, indicating alterations in amino acid sequence. We plan to study the amino acid sequence of a prototype hemagglutinin, Heql. Comparison of the sequence of this antigenically distinct type A influenza virus hemagglutinin with the sequences obtained for the H2 and H3 subtypes of hemagglutinin will permit us to determine the precise changes in amino acids which have occurred and may be related to changes in antigenicity. Knowledge of the primary sequence of the surface antigens may eventually permit synthesis of the antigenically significant sites to produce vaccines which are effective against all major influenza subtypes. When influenza virus is cultivated in ovo, the hemagglutinin polypeptide is cleaved asymmetrically with the separate chains held together by disulfide bonds. This characteristic of the hemagglutinin structure permits us to separate the polypeptides (HA1 and HA2) from each other as well as from other major viral proteins in milligram quantities by SDS gel chromatography under non-reducing followed by reducing conditions. The viral proteins can be separated from laboratory viral preparations or vaccine lots for both type A and type B influenza viruses. Preliminary study of the amino terminal sequences of the Heql hemagglutinin polypeptides (HA1 and HA2) has shown cyclical repeat of glycine at intervals of three to four residues from the amino-terminus of HA2 through the first 24 residues. CNBr cleavage of the HA1 and HA2 polypeptides has been performed and the fragments separated. Amino acid compositions of isolated CNBr fragments have been determined. Analysis of amino acid sequence for these fragments is now under investigation.